1. Field of the Invention
The present invention relates to an inorganic EL element capable of emitting light with direct current.
2. Description of the Related Art
Electroluminescence (referred to as “EL” in the present invention) elements are roughly classified into inorganic EL elements and organic EL elements due to the difference of their light emitter materials. The inorganic EL element using an inorganic material for the light emitter has a characteristic that the emission lifetime is longer than that of the organic EL element using an organic material for the light emitter. Thereby the inorganic EL element has been put to practical use mainly for applications requiring high durability, such as cash register display units, in-vehicle monitors, and clock backlights.
FIG. 1 is a perspective view showing a key portion of a representative configuration of a conventional inorganic EL element. An EL element 10 is a thin film EL element of double insulation type, which is formed by laminating lower electrodes 12, a lower insulator layer 13, a light emitter layer 14, an upper insulator layer 15 and upper electrodes 20 in the described order on a transparent substrate 11 having an electric insulation property (see FIG. 3 of JP-A No. 2004-265740).
As the transparent substrate 11, there is employed a transparent substrate such as a blue plate glass commonly used in LCD (Liquid Crystal Display) and PDP (Plasma Display Panel). The lower electrode 12 is generally composed of ITO (Indium Tin Oxide) with a film thickness of about 0.1 to 1 μm. The upper electrode 20 is composed of a metal such as Al. The lower insulator layer 13 and the upper insulator layer 15 are thin films with a thickness of about 0.1 to 1 μm each formed by a method such as sputtering or vacuum evaporation, which are generally made of Y2O3, Ta2O5, AIN, BaTiO3 and other materials. The light emitter layer 14 is generally composed of a light emitter containing a dopant which is to be an emission center, and its film thickness is generally about 0.05 to 1 μm.
In the conventional EL element having such a configuration, when the electrodes are applied with an alternative voltage or a pulse voltage from an AC power source 21, the light emitter layer 14 produces electroluminescence, and its output light is taken out from the transparent substrate 11 side. In the use of the inorganic EL element as a display, the lower electrodes 12 and the upper electrodes 20 are provided in a stripe-like pattern, wherein one side is set to the row electrodes and the other side is set to the column electrodes, and both of the electrodes are arranged so that the stretching directions thereof are orthogonal to each other. In other words, the matrix electrodes are formed by the electrodes 12 and the electrodes 20, in which the portions of the light emitter layer at the crossovers of the row and column electrodes represent pixels, thereby when the alternative voltage or pulse voltage is selectively applied thereto, a specific pixel is emitted, and its output light is taken out from the transparent substrate 11 side.
However, the above inorganic element requires to be applied with an alternative voltage of 100 V or more at a frequency in the range of several hundreds Hz to dozens KHz, being less and less used in mobile devices such as notebook computers and cellular phones that use batteries, due to the fact that the DC-AC conversion element is necessary and for other reasons.
On the other hand, attention has been recently focused on the organic element as an element capable of being driven by a direct current, which is commercialized for the in-vehicle applications and cellular phones that use batteries. However, a fluorescent organic solid which is a material of the light emitting layer of the organic EL element is weak against moisture, oxygen and other substances. Also, the characteristics of the electrode provided on the light emitting layer directly, or through a hole injection layer or electron injection layer, are likely to deteriorate due to oxidation. Thus, there is a problem that when the conventional organic EL element is driven in the atmosphere, the emission characteristic deteriorates rapidly. Various efforts are made to cope with this problem (for example, see JP-A No. Hei 11-329718), but the problem is not perfectly solved from the point of view of the lifetime.
Thus, the direct current inorganic EL element capable of being driven by direct current has been studied with taking advantage of the feature of the long lifetime of the inorganic EL (for example, JP-A No. Hei 5-74572, JP-A No. 2002-313568).
However, in the case of the conventional direct current inorganic EL elements including those described in the above Patent Documents 3 and 4, mostly there is no description about the brightness, and the approach to realize high brightness has not been sufficiently carried out.